1. Field of the Invention
The present invention relates to spark plugs for use in internal combustion engines. More particularly, the present invention relates to wear-resistant electrode tips for spark plugs, and to spark plugs containing such wear-resistant tips.
2. Description of the Background Art
Spark plugs are widely used to ignite fuel in internal combustion engines. Spark plug electrodes are subject to intense heat and to a highly corrosive environment generated by the exploding air/fuel mixture. To improve durability and erosion resistance, spark plug electrode tips must be able to withstand the high temperature and corrosive environment resulting from the chemical reaction products between air, fuel, and fuel additives within a combustion chamber.
SAEJ312 describes the specification for automotive gasoline used as a fuel in the United States. The gasoline consists of blends of hydrocarbons derived from petroleum: 50-80 percent saturates, 0-15 percent olefins, and 15-40 percent aromatics. Leaded gasoline contains about 0.10 grams of lead per gallon of fuel (0.026 g Pb/liter), and 0.15 percent sulfur. In unleaded gasoline there is about 0.05 grams of lead per gallon (0.013 g Pb/l), 0.1 percent sulfur, and 0.005 g phosphorous per gallon (0.0013 g P/liter).
In addition, there are a number of additives incorporated into gasoline for various reasons. For example, tetramethyllead (TML) and tetraethyllead (TEL) are added as antiknock agents. Carboxylic acid compounds such as acetic acid are added as lead extenders. Aromatic amines and phenols are added as antioxidants. Organic bromine and/or chlorine compounds are added as scavengers and deposit modifiers. Phosphors and boron-containing compounds are added to reduce surface ignition, preignition, and as engine scavengers. Metal deactivators are added to reduce oxidative deterioration of fuel by metals, such as Cu, Co, V, Mn, Fe, Cr and Pb. In addition, carboxylic acids, alcohols, amines, sulfonates, and phosphoric acid salts of amines are used as rust-inhibiting additives.
The manufacture of copper (Cu) and nickel (Ni) electrodes for spark plugs is a proven art and has been acccomplished in various ways. For instance, U.S. Pat. No. 3,803,892 describes a method of producing extruded copper and nickel electrodes from a flat plate of the two materials. U.S. Pat. No. 3,548,472 discloses a method of cold-forming an outer nickel cup-shaped sleeve in several steps, inserting a piece of copper wire into the cup, and then lightly pressing the two materials together. U.S. Pat. No. 3,857,145 discloses a process for making a spark plug center electrode in which a central copper core is inserted into a nickel member and attached thereto by a collar portion, to assure that an electrical flow path is produced.
Platimum-4% tungsten (thoriated) alloy was originally developed for spark plug electrodes in aircraft engines by INCO in 1939-1940 to replace platinum-iridium alloys as there was a world shortage of iridium.
The use of certain types of welded-on spark plug electrode tips, which are more wear-resistant than the body of the electrode, is also known. Some of these known welded on spark plug electrode tips are disclosed, for example, in U.S. Pat. Nos. 4,810,220, 4,840,594, and 5,456,624.
Although the use of wear-resistant spark plug electrode tips is known, a need still exists in the art for a spark plug electrode tip made of an improved composition having superior wear-resistant properties